1. FIELD OF THE INVENTION
The present invention relates generally to a constituent member having a high strength and a high wear resistance and a method of producing the same, as well as a valve gear using the same for use in an internal combustion engine. More particularly, the present invention relates to a composite member composed of a wear-resistant material suitable for use in forming sl:ding members subjected to high loads or impact loads. The invention further relates to a method of producing such a wear-resistant member, and its use.
2. DESCRIPTION OF THE PRIOR ART
In the field of structural components, it is generally unnecessary to ensure that the whole of each structural component is provided with certain properties required for specific purposes. In a typical case, the greater part of a structural component is composed of a relatively inexpensive material, but a specific portion of the surface of the structural component requires particular properties. For instance, a cutting tool is normally constituted by a combination of a hard cutting portion and a remaining portion made of a material which is strong enough not to be deformed or broken by the cutting load. In addition, as the size of such a component increases, a proportion of the part in the component occupied by the portion requiring specific properties is often relatively reduced. It is therefore advantageous, in terms of performance and price, to form such a component by a combination of a base material occupying the greater part of the component and a surface layer made of a material having desired properties. In particular, composite members comprised of a base material coated with a hard surface layer are employed as sliding components of the type which requires a certain level of wear resistance.
Such a composite member for use as a sliding component is described, for example, in Japanese Patent Publication No. 12424/85 which discloses a composite member comprised of a base material which is plasma-sprayed with a powder of high carbon--high Cr cast steel or a mixture of that powder and a powdered self-fluxing alloy. Further, Japanese Patent Publication No. 12425/85 discloses a composite sliding member comprised of a base material which is plasma-sprayed with a powder of high carbon--high Cr cast steel and a powder of Cu alloy. In the process of manufacturing either of these prior-art composite members, however, plasma spraying is effected under atmospheric pressure conditions. Accordingly, it is impossible to achieve satisfactory adhesion of the sprayed material to the base material, as well as a sufficient adhesion strength between individual layers of particles contained in the resultant coating. In addition, no investigation has been made on the density of precipitated hard intermediallic compounds and the degree of dispersion thereof.
Further, Japanese Patent Publication No. 57552/82 discloses a method of using CVD to coat a base material with a layer of a precipitated hard metal alloy composed of a metal halide and carbon, boron or silicon. This method utilizing CVD, however, involves a problem in that the strength of adhesion between the base material and the layer or the toughness of the precipitated layer is reduced because of treatment strains caused by differences in physical values between the base material and the layer coated thereon, since the precipitated layer is present in a single phase. The above Publication further discloses that only the precipitated layer is utilized by taking it out. However, as the size of the precipitated layer increases, it becomes impossible to achieve a sufficient toughness, because the precipitated layer is an intermetallic compound. Also, since heat decomposition of the metal halide is utilized to form the precipitated layer on the base material, the treatment cost per unit area increases due to various factors such as the high production cost of the metal halide and the necessity for post-treatment of a halogenating gas. This limits the kind of components to which this prior art method is applicable.
On the other hand, for a high hardness material (ingot) made by melting, an alloy disclosed in Japanese Patent Publication No. 17069/82 is known as a wear-resistant cutting tool steel. As the content of MC-system carbide is increased, the wear resistance of this alloy is improved. However, if the V content is increased in order to increase the MC-system carbide content, the melting temperature of this alloy rises, thereby making it difficult to produce the alloy. In addition, the specific gravity of the MC-system carbide is lower than that of the melt, so that the MC-system carbide tends to move upward during melting, and this hinders the production of a homogeneous metal structure. Moreover,as the melting temperature rises, the particle size of carbide becomes larger during the crystallization thereof, thereby causing reductions in toughness and in machinability. Therefore, in a melting method, the composition range of the alloy is determined by the conditions governing working, not by the properties of a product, thereby reducing the range of machine design.
It is known that a valve gear incorporated in an internal combustion engine has various sliding surfaces which are maintained in sliding contact with each other, and the sliding surfaces thereof are made of alloy steel or case-hardened steel which is subjected to surface hardening by means of heat treatment. In this case, a thickly hardened layer or a hard sintered material is embedded in a portion of a cam shaft which is in contact with a cam wheel, since that portion requires an extremely high wear resistance. For example, Japanese Patent Application Laid-Open Publication No. 53612/83 discloses a structure in which a Co-based sintered alloy containing carbide is bonded, at the surface of a tappet contacting with a cam, to a body of the tappet made of steel or cast iron through an intermediate layer consisting of Fe-based sintered alloy which was sintered in liquid phase. The valve lifter (called "tappet" in the above Laid-Open Publication) possesses a very good wear resistance, such as scufing resistance, etc. However, in the production of the structure, the Co-based alloy powder used for the surface layer is compacted and then the Fe-based alloy powder to be sintered in liquid phase is compacted thereon, and thereafter they are attached to the body of the valve lifter. Then, the thus-assembled body is heated to a temperature at which the Fe-based sintered alloy becomes liquid phase. Accordingly, in this production process no satisfactory considerations are given to a productivity, a deformation caused by the heating to high temperatures, and an increase in the price incurred by the use of expensive materials such as Co.
On the other hand, Japanese Patent Application Laid-Open Publication No. 214609/83 discloses a valve lifter in which a reduction in the weight is taken into consideration. According to the art disclosed in this Laid-Open Publication the body of the valve lifter is produced from a casting of aluminum, magnesium or other light alloys, and the sliding portion of its surface which is brought into contact with a cam wheel is sprayed with ceramics, tungsten carbide or the like. Accordingly, a reduction in the weight of the body is achieved to some extent, but the wear resistance and the durability of the surface are not sufficiently taken into consideration. In a typical spraying method, particles having a particle size of several .mu.m to several hundreds .mu.m are sprayed onto a base material to form a coating thereon. Accordingly, the bonding strength between the coating and the base material is achieved mechanically, and the strength thereof will be several kg/mm.sup.2 at best. Also, the interior of the coating exhibits a laminated structure containing a multiplicity of pores, and thus the bonding strength between individual layers formed by the sprayed particles is weak. Therefore, the phenomenon of pitting may take place under conditions of high-load friction. In addition, the body does not have a sufficient toughness since it is formed from a light alloy casting.